Transient cultivation and fallow land in forests. Measurements of the specific activity of the carbon in organic matter fractions for the purpose of studying the renewal of the store of organic matter in an equatorial forest environment

Different conditions of forest-type fallow land and of clearings in the equatorial forests of the Amazon region provide a basis for determining the nature, the direction and the magnitude of the transformations of the organic matter of the soil when cultivated, using the variations in the specific activity of atmospheric carbon. Calculations of the renewal rates of surface horizon organic matter show that there are two groups of products, (a) humin, a labile fraction with a high renewal rate, and (b) humic and fulvic products with a lower renewal rate. The effect of clearing is generally to retard these renewal rates. The dynamics which become established in forest fallow land mainly involve the fulvic products, and the return to forest equilibrium can be considered to occur between 60 and 100 years after clearing. (author)